Prepare sauce: Coarsely chop the onion and sautee it in the olive oil with the red pepper flakes. Add salt and pepper.
Add two sticks of butter and let them melt. Add the wine and lemon juice.
Light the fire and let it burn to coals. While it's burning down, sort and clean the shellfish, discarding any damaged or dead ones. (If they're open, tap them. If they don't close, they're dead. If they're closed, they're fine.)
Lay shellfish on grill until they pop open. The hotter the fire, the shorter the time it will take - five minutes or more.
Add shellfish to sauce and stir to mix.